The present invention relates generally to data communications. More particularly, the present invention relates to multicast bridging in network switches.
A packet in a packet-switching network such as an Ethernet network can be sent in one of three ways: unicast, multicast, and broadcast. A unicast packet is directed to a single port. A broadcast packet is directed to all of the ports of the network. A multicast packet is directed to a group of the ports of the network. Multicast packets can be either link-layer multicast packets, such as media access control (MAC) multicast packets, or Internet Protocol (IP) multicast packets.
Ethernet switches compliant with the IEEE 802.1D-1998 specification are required to switch MAC multicast packets based on the destination MAC address of the packet. Ethernet switches compliant with the IEEE 802.1D-1998, 802.1p and 802.1q specifications are required to switch MAC multicast packets based on the destination MAC address and Virtual Local Area Network (LAN) Identifier (VLAN ID) of the packet. Such switches comprise forwarding databases (FDB) that associate each MAC address (or MAC address and VLAN ID combination) with one or more of the ports of the switch.
However, most multicast packets are IP multicast packets, which should be flooded according to the source IP address and IP multicast destination (also referred to as IP multicast group). The Internet Engineering Task Force (IETF) mandates that IP multicast traffic should be encapsulated in a link-layer MAC multicast packet when sent over Ethernet. Unfortunately, Ethernet switches handle IP multicast packets inefficiently, without regard to the source IP address, according to the MAC destination address only. And because a MAC destination address can map to more than one IP multicast group (32 unique IP multicast addresses map to each MAC multicast address), an IP multicast packet is often flooded to more than one IP multicast group, causing unnecessary network traffic, security breaches, and switch workload.
For example, consider a switch having four ports p1, p2, p3, and p4. Assume that two IP multicast groups, IPMG1 and IPMG2, are mapped to a single MAC multicast destination address MMDA, that IP multicast group IPMG1 is mapped to ports p1 and p2, and that IP multicast group IPMG2 is mapped to ports p3 and p4. Therefore MAC multicast destination address MMDA is mapped to all four ports. When an IP multicast packet arrives for IP multicast group IPMG1, it is flooded not only to ports p1 and p2, but also to ports p3 and p4. Similarly, IP multicast packets for IP multicast group IPMG2 are flooded to all four ports.
A similar problem occurs in Ethernet switches that comply with the Internet Group Membership Protocol (IGMPv3). According to the IGMP protocol, a port can ask to receive all of the traffic sent from an IP source address to an IP multicast destination address. However, because Ethernet switches forward IP multicast packets without regard to the source IP address, the port will receive all of the traffic sent to that IP multicast destination address from any IP source address, thus unnecessarily burdening the port and the network.